About fostering in the uk?

im 20 years old and since doing about fostered children in school when i was 13 yrs old its something ive always wanted to do. I know you have to be 21 to foster but i was wondering if anybody knew if you could start the proceedings before you were 21 so that once you turned 21 you were ready to start. I know some people may think 21 is a lil young but im a very mature 20 year old and all i want to do is help children and their families where im needed. Ive experience with children as i have my own. Thanks in advance for your answers. If your going to criticise my wanting to help others then dont waste mine or your time replying thank you.